1. Field of the Invention
The present invention relates generally to lasers diodes and, in particular, to a package that is resistant to sudden short-circuit failures and resistant to sudden open-circuit failures.
2. Discussion of the Related Art
Semiconductor laser diodes have numerous advantages. They are small in that the width of their active regions is typically submicron to a few microns and their height is usually no more than a fraction of a millimeter. The length of their active regions is typically less than about a millimeter. The internal reflective surfaces, which produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability. The laser diode typically has several emitters, each of which is aligned with a corresponding active region.
High efficiencies are possible with semiconductor laser diodes with some pulsed junction laser diodes having external quantum efficiencies near 50%. Semiconductor lasers produce radiation at wavelengths from about 20 to about 0.7 microns depending on the semiconductor alloy that is used. For example, laser diodes made of gallium arsenide with aluminum doping (AlGaAs) emit radiation at approximately 0.8 microns (˜800 nm) which is near the absorption spectrum of common solid-state laser rods and slabs made from Neodymium doped, Yttrium-Aluminum Garnet (Nd:YAG), and other crystals and glasses. Thus, semiconductor laser diodes can be used as the optical pumping source for larger, solid-state laser systems.
Universal utilization of semiconductor laser diodes has been restricted by thermal related problems that can cause catastrophic failures. These problems are associated with the large heat dissipation per unit area of the laser diodes which results in elevated temperatures within the active regions and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode is decreased as the operating temperature in the active region increases.
In particular, laser diode bars containing more than one emitter are vulnerable to a sudden short-circuit failure. This short-circuit failure begins when one of the output facets of an emitter begins to absorb optical energy or when heat is not sufficiently dissipated from the emitter. As the temperature rises, the emitter becomes more inefficient and absorbs even more heat, causing a further rise in temperature and, ultimately, a thermal runaway situation. The temperature may reach levels that cause the material of the laser diode bar to melt in the area of the associated active region. Once the melting has occurred, the current and voltage characteristics of the P-N junction at the active region are locally destroyed and, thus, the active region begins to act as a simple resistor. With the P-N junction locally destroyed, current that would normally be distributed equally among all the active regions rushes through the damaged area, depriving the rest of the active regions of some or all of the available current. If the damaged region or regions are large, then it is possible for all of the available current to flow through the damaged region, and the rest of the undamaged active regions and their associated emitters on the laser diode bar become nonfunctional. Thus, an array containing the damaged laser diode bar may continue to draw current, but will have an inadequately low amount of emission or no emission whatsoever.
Additionally, laser diode bars are also vulnerable to sudden open-circuit failure. This failure mode may begin as a short-circuit failure. The emitter that has inadequate heat extraction causes the entire laser diode bar to become heated. The heat causes the soldered electrical connections between the laser diode bar and the adjacent heat sink to melt and the bar delaminates from the adjacent heat sink. Once the separation has occurred, an electrical connection no longer exists, thereby forming an open circuit. This mode of failure is more common in a diode package in which the diode bar is in intimate contact with a solid foil or a ribboned foil as the heat sink. The destruction of the solder bond between the foil and the laser diode bar forces the package into an open-circuit condition.
A need exists for a laser diode package that is not susceptible to short-circuit and open-circuit failures.